Transition to bring overlay into view is making mov background go lighter but not mp4

Question

So I have an animated background, I use slide to bring something into view, its not taking up the entire screen, just an image sliding in and back out again on one side of the screen, when the slide animation is happening the background (a mov file) become brighter, then goes darker again once the slide in is finished.

I tries converting a mov file to a mp4 and put that in instead and the lightning of the background didn't happen when I slid in the sane png file (though the converted file appears to be permanently a little lighter, animation or not which I'm not sure why at the moment but I'll stick with that and just make it darker again to suit.

Anyone else ever experienced this or know why it is?

Answer 1

I think I know the answer, the main difference between the file types is that the mov file actually had its opacity reduced below 100% and that seems to be a factor, TIL...